A quick search suggests that insofar is the American variant of the British in so far. I always assumed it belonged to the set of expressions like hitherto, heretofore, therefore and albeit. Is there a preferred usage, and does it matter? General context for the usage of these expressions would be helpful.
The form insofar is American English; the standard form in British and other English is 'in so far'.
I also have never seen the form 'in so far' used in American English.
I am from USA. where insofar is prevailing. I actually haven't encountered "in so far" at all.
I can only think of context using insofar as or insofar that. It seems to be a wordy way to say since or because of For example: Insofar as we received 24 inches of snow, people are advised to limit their driving as much as possible.